Foils are produced by a variety of methods according to the materials or projected uses of the foils. A representative foil production method is a rolling method, in which metal is passed between pressure rollers while being pressed by the rollers to thereby roll the metal into a foil.
In this method, the rolling speed increases as the metal thickness decreases during the rolling operation. This production method, therefore, is suitable for mass production. However, the rolling method is disadvantageous in that production of thinner foils requires far more complicated techniques and poses problems concerning application of tension, roller nip control, etc. Further, rolled foils do not always have a uniform thickness in the width direction because of limitations on the shapes of the rollers employed in the operation.
On the other hand, electrolytic copper foils, particularly those used in copper-clad laminates to be used as printed-circuits, have been recently investigated. One exemplary apparatus for electrolytic copper foil production is illustrated in FIG. 5. In this method, an electric current is applied between a large-sized cathodic roller 53 as the cathode and an insoluble anode 54 as the counter electrode, with the lower part of the cathodic roller 53 being immersed in an electrolyte solution 52 in an electrolytic tank 51, thereby to continuously plate the roller surface with copper, and simultaneously, the deposited copper metal 55 is continuously peeled from the roller surface to obtain a copper foil 56. This electrolytic method is characterized in that the average thickness of the copper foil 56 being produced can be easily controlled by changing the intensity of the electric current applied, and this method can easily produce thin foils unlike the rolling method described above.
However, the above-described electrolytic foil production is disadvantageous since the distribution of applied current becomes nonuniform when the electrodes used in the electrolysis process are consumed or undergo a change in their electrochemical properties, and this may result in the copper foil being produced having different thicknesses in its width direction. Of course, this unevenness in thickness can be easily corrected by masking the electrode surfaces or by other means, unlike the thickness nonuniformity encountered in the mechanical foil production by means of pressure rollers.
In foil production, various techniques have conventionally been used for measuring foil thickness and In some instances, foil thickness is controlled automatically.
For example, in the mechanical production process for a rolled foil, a representative method is to continuously gauge the foil thickness with a micrometer. That is, the thickness is measured with a contact-type micrometer during rolling. According to this technique, the thickness of a foil is continuously measured only in its portions near the side edges because using the contact-type thickness meter may scar the foil, and the results obtained by the gauging are utilized for controlling the roller nips and tension. Although thickness measurement over the foil width cannot be made in such a method, this, as a practical matter, is not a problem because in foil production by mechanical rolling, the thickness distribution in the width direction is almost constant throughout the production.
Thickness meters utilizing X-rays or .gamma.-rays are also being used in which an X-ray or .gamma.-ray source is placed on a first side of a foil and a detector is placed on a second side. However, since such a thickness meter, like the contact-type micrometer, has been designed for use with rolled foils, it is being utilized to determine the thickness distribution in the rolling direction of the foil, not in the width direction of the foil. That is, the thickness meter employing X-rays or .gamma.-rays has been used to sense a thickness change in the rolling direction to control the rolling device. Although rolled foils, in particular, tend to have larger thicknesses around both side edges thereof than in their central portions because of the pressure rollers which generally have larger diameters at their central portions, such rolled foils each have an almost constant thickness distribution in the width direction. Hence, the thickness meter utilizing X-rays or .gamma.-rays has been used only to gauge the thickness of the foil along the conveyance direction of the roll to control the average thickness based on the data obtained.
As described above, the foil thickness-measuring techniques have been used in the production of rolled foils to control the average thicknesses of the foils. However, in producing electrolytic foils, although average thickness control can be easily attained because average thickness is proportional to electric current employed for electrolysis, the thickness of a foil being produced changes depending on the electrochemical properties of the surfaces of the electrodes, i.e., the counter electrode for electrolysis and the cathodic roller on which a metal is electro-deposited. Thus, if the electrode properties become uneven, the resulting foil has different thicknesses in the width direction. However, none of the conventional devices have been able to precisely determine the thickness distribution of a foil in the width direction.
On the other hand, there is a device for continuously measuring and controlling the thickness of a metal layer formed by plating. That is, the device automatically measures the thickness of a deposited zinc or tin layer formed by plating on a steel plate and controls the amount of metal being deposited. For plated articles with small deposited metal thicknesses, an X-ray fluorescence spectroscopic technique is usually employed which enables precise gauging. This technique has an advantage that the unevenness of the thickness in the width direction can also be determined by making a measurement with the gauging head traversing the width direction of the plated article, but it necessitates a large-capacity X-ray source and an expensive X-ray spectrometer. Further, this technique is defective in that sufficient determination of thickness distribution cannot be conducted because the device has been designed primarily for the measurement of the composition of the deposit and the geometrical relationship between the device and the work (deposit) must be strictly controlled which is difficult for the foil producing. Further, the deposit thicknesses which can be measured by this technique are limited to relatively small values, i.e., 10 .mu.m or less, although such a range varies depending on the types of plated articles produced.
Other devices for thickness measurement include a .beta.-scope which employs a radioisotope and utilizes .beta.-ray backscatter. Although this device can be used to measure the thickness of relatively thick materials, the device does not have the measurement precision required for the thickness measurement of electrolytic copper foils for use in, for example, printed-circuits.